1. Field of the Invention
The present invention relates to a friction detecting device for vehicles, particularly to a device for detecting a frictional coefficient between vehicle wheels and a road on which the vehicle runs, more specifically to a device for detecting a dynamic frictional coefficient which changes in accordance with vehicle operating condition.
2. Description of the Prior Art
There has been proposed a friction detecting device for detecting a frictional coefficient between vehicle wheels and a road surface and controlling a steering of a vehicle in accordance with a result of the detection because a frictional force between the wheels and the road surface affects a vehicle driving property significantly.
Japanese patent public disclosure No. 59-148769 discloses a vehicle steering system including a device for setting plural values of frictional coefficient between vehicle wheels and a road surface in accordance with a steering angle for front wheels, calculating lateral accelerations against the vehicle corresponding to the values of the frictional coefficient, comparing the calculated values of the lateral acceleration with an actual value of the lateral acceleration detected, presuming a fictional coefficient corresponding to a calculated value of the lateral acceleration closest to the actual value of the lateral acceleration detected as a desirable frictional coefficient, and carrying out a steering control taking account of the desirable frictional coefficient.
It should however be noted that the steering system disclosed in the above Japanese application does not consider a dynamic property or a transient response property such as a vehicle lateral acceleration and a yawing rate produced in the vehicle which are changed in accordance with a steering amount applied to the vehicle wheels, and that the steering system has nothing but the way to presume the frictional coefficient constant. Therefore, although the steering system can infer the frictional coefficient precisely when the steering angle is constant, for instance, the vehicle runs on a true circular path, as a matter of fact, it is unlikely that the vehicle runs frequently with a constant steering angle. Thus, the steering system as disclosed in the Japanese patent public disclosure No. 59-148769 cannot get a precise value of the frictional coefficient actually so that it is difficult to accomplish a desirable and precise steering control in accordance with the vehicle operating condition.
U.S. Pat. No. 4,733,878 owned by the same assignee as the present application discloses a four wheel steering control system in which a steering ratio of the rear wheel steering angle to the front wheel steering angle is controlled to a same phase direction when the frictional coefficient is small. In this patent, the frictional coefficient is obtained through a similar procedure to the Japanese public disclosure No. 59-148769 so that the dynamic property of the vehicle or the transient property is not taken into account for the control. This means that the control disclosed in the U.S. Patent has the same problem as the Japanese application aforementioned.